Extension Cord Gauge Calculator
Compare estimated voltage drop for common copper conductor sizes based on load current, one-way cord length, and supply voltage. This is a voltage-drop estimate only, not a safe ampacity rating.
How this calculator works
This tool compares calculated voltage drop using approximate copper conductor resistance for common AWG sizes and a round-trip path. It does not determine a safe current rating, temperature rise, insulation rating, plug/receptacle compatibility, duty cycle, or suitability for outdoor or wet locations. Those limits depend on the complete cord assembly and its manufacturer rating.
estimated drop (V) = current (A) × round-trip length (ft) × conductor resistance (Ω/1,000 ft) ÷ 1,000 voltage drop (%) = estimated drop ÷ supply voltage × 100 select the smallest listed conductor size that meets the entered drop limit
Worked example
At 10 A over a 50 ft one-way cord on 120 V, a 14 AWG copper conductor (about 2.525 Ω per 1,000 ft) gives an estimated 2.5 V drop, or about 2.1%. This is only a voltage-drop comparison, not proof the cord is suitable for the load.
Use the cord and equipment ratings
Choose a cord specifically rated for the equipment, current, environment, and intended use. Check the appliance instructions and cord label; do not rely on this estimate to justify using a cord with an inadequate ampacity or damaged insulation. When requirements are unclear, ask a qualified electrician or the equipment manufacturer.
Understand the calculation assumptions
The calculation assumes copper conductors and uses approximate resistance values; actual resistance varies with conductor temperature and construction. The entered length is the one-way cord length, doubled internally for the outgoing and return conductors. The gauge result is a voltage-drop comparison only and is not an electrical-code or safety recommendation.
Frequently asked questions
What extension cord gauge do I need?
It depends on the appliance, current, cord length, voltage, and the cord assembly rating. This calculator compares approximate voltage drop for common copper sizes only; always follow the equipment and cord manufacturer ratings.
Why does cord length affect voltage drop?
Current travels through both the outgoing and return conductors, so a longer cord has more conductor resistance and can produce more voltage drop under load.
Does this calculator tell me the safe amp rating?
No. It calculates voltage drop only. It does not establish safe ampacity, temperature rating, construction quality, or whether a cord is appropriate for a specific tool or environment.
Can I use a smaller gauge if the estimated drop is low?
Not based on this estimate alone. The cord must also meet the current, temperature, environment, and equipment requirements listed by its manufacturer and applicable electrical rules.